Update Ubuntu 20.04 image

[mbrukman]

The ci.yml config is currently referencing ubuntu-20.04 (while macOS and Windows are "latest"):

pytype/.github/workflows/ci.yml

Line 16 in 92251b8

os: [ubuntu-20.04, macos-latest, windows-latest]

In the meantime, build.yml config refers to ubuntu-latest:

pytype/.github/workflows/build.yml

Line 18 in 92251b8

runs-on: ubuntu-latest

Would it make sense to (gradually?) update Ubuntu version in ci.yml to Ubuntu 22.04 / 24.04 / latest to bring it up-to-date? At some point, Ubuntu 20.04 will be deprecated and removed from GitHub Actions, so we can't keep it indefinitely.

[h-joo]

Thank you for the report!
I've addressed some parts of this issue in #1893. (there's still discrepancy between build.yml and ci.yml but I'll try to address this later.)